In recent years, liquid crystal display devices have been used in various fields and have been used under various environments. Liquid crystal display devices are also used, for example, under a high temperature and humidity environment. However, when a liquid crystal display device is used under a high temperature and humidity environment, characteristics of a thin film transistor (hereinafter, referred to as a TFT) changes due to the influence of moisture that enters inside the device, which may cause display degradation.
FIG. 16 is a cross-sectional view of a conventional liquid crystal display device. The liquid crystal display device shown in FIG. 16 has a structure in which a TFT substrate 81 and a counter substrate 82 are bonded together and a liquid crystal 83 is filled between the two substrates. A seal 84 is provided around a portion where the two substrates face each other. The TFT substrate 81 is obtained by forming a TFT 86 on a glass substrate 85 and forming a protective film 87 and a flattening film 88 thereon.
For the flattening film 88, for example, an acrylic resin film is used. However, the flattening film 88 made of an acrylic resin has high moisture absorption properties. Due to this, when the liquid crystal display device is used under a high temperature and humidity environment, moisture in the air enters the flattening film 88 from the neighborhood of the seal 84 (see arrow A1). The moisture that enters spreads out in the flattening film 88 and reaches near the TFT 86. Although the TFT 86 is covered with the protective film 87, since a surface of the TFT 86 is uneven, it is difficult to completely cover the TFT 86 with the protective film 87. Hence, the moisture may pass through the protective film 87 and reach an interface between the protective film 87 and a semiconductor layer 89. At this time, characteristics of the TFT 86 greatly changes due to the influence of the moisture.
Patent Document 1 describes liquid crystal display devices for solving this problem (see FIGS. 17 and 18). In a liquid crystal display device shown in FIG. 17, the edge surfaces of a flattening film 91 are covered with a seal 92. In a liquid crystal display device shown in FIG. 18, the edge surfaces of a flattening film 91 are covered with a protective film 95. As such, Patent Document 1 describes the liquid crystal display devices configured to prevent direct contact of the flattening film 91 with air.